Method for recovering methylnaphthalenes



Dec. 31, 1963 c. s. SHEPPARD ETAL 3,115,341

METHOD FOR RECOVERING METHYLNAPHTHALENES Filed May 4, 1961 FUR/FICA T10COLUUIIS CONTAINER 28 cars nu 12m ISOMERIZA T10 mu Man 29 [IE4 TERlA/VE/VTORS 01155755 3. SHEIfPARD and cLA/R J. WARN/N6 Alfarney UnitedStates Patent 3,116,341 METHOD FOR RECOVERING METHYL- NAPHTHALENESChester 5. Sheppard, Edgewood Borough, Allegheny County, and Clair J.Warning, Plum Borough, Allegheny County, Pa., assignors to United StatesSteel Corporation, a corporation of New Jersey Filed May 4, 1961, Ser.No. 107,795 6 Claims. (Cl. 260674) The present invention relates to amethod for recovering methylnaphthalenes from coal tar, and moreparticularly to the recovery of Z-methylnaphthalene, useful as ananthelmintic drug and in the synthesis of vitamin K.

Methylnaphthalenes are present in coal tar as relatively minorconstituents. A typical coal tar may contain about 1 percent ofl-methylnaphthalene and about 1.5 percent of Z-methylnaphthalene. A 230to 260 C. cut, subsequently acid washed, may contain about 30% to 40%methylnaphthalenes. Known methods for the recovery of methylnaphthalenesfrom coal tar give low yields and involve many tedious and costly steps.As a result, the recovery of methylnaphthalenes from coal tar has not,to our knowledge, been practiced on a commer cial scale. Accordingly, itis an object of our invention to provide a method wherebymethylnaphthalenes of relatively high purity may be recovered from coaltar economically, in good yield, and on a substantial scale.

According to a preferred practice of our invention we 1) Distil anacid-washed 230 to 260 C. cut of coal tar at atmospheric pressure toremove, as heads, impurities boiling below about 237 C.;

(2) Distil the cut further at subatmospheric pressure to removehigh-boiling impurities, thereby preventing or minimizing formation ofazeotropes and obtain a fraction containing about 90% mixedmethylnaphthalenes;

(3) Chromatographically remove nitrogen and others of the remainingimpurities;

(4) Crystallize out Z-methylnaphthalene;

(5) Isomerize the mother liquor from the crystallization,

which mother liquor is relatively rich in l-methylnaphthalene, toprovide additional 2 -methylnaphthalene;

(6) Recycle the isomerized mother liquor to the crystallizer to providefor the removal of the additional 2- methylnaphthalene.

A complete understanding of the invention may be obtained from thefollowing detailed description and we planation which refer to theaccompanying drawing illustrating the present preferred practice. Thesingle figure of the drawing is a diagrammatic representation of asystem for carrying out the method of our invention.

Referring in detail to the drawing, an acid-washed, coal tar fractionboiling between about 230 and 260 C. and containing between about 30 and40 percent methylnaphthalenes, flows through pipe 10 to a distillationcolumn 11 having 20 to 40 theoretical plates, preferably 40. The columnmay be operated at a reflux ratio of 1:1 to 5:1, preferably 5:1, toremove as heads a fraction boiling under about 237 C. The higherboilingbottoms flow from the bottom of column 11 through pipe 12 to adistillation column 13. Column 13, unlike column 11,, which may beoperated at substantially atmospheric pressure, is operated at a totalpressure of less than 250 mm. of mercury, preferably between about 15mm. and 50 mm. mercury absolute pressure. Column 13 may have to 40theoretical plates, preferably 40, and may be operated at a reflux ratioof 1:1 to 5:1, preferably about 5: 1, whereby there is obtained a cutboiling up to about 145 C. when the pressure is 50 mm. of mercuryabsolute.

3,116,341 Patented Dec. 31, 1963 The described successive atmosphericand vacuum distillations are required to separate a product comprisingabout 90% methylnaphthalness in high yield, for example, at leastpercent recovery of the methylnaphthalenes originally present in thecoal tar. This is so because the crude fraction boiling between about230 and 260 C. contains numerous compounds, with many azeotropes andpolyazeotropes present when such a fraction is distilled at atmosphericpressure. A number of these compounds do not form azeotropes atsubatmospheric pressures. The initial heads removal to about 237 C. atatmospheric pressure, carries along some impurities as lower-boilingazeotropes that would boil with the methylnaphthalenes at subatmosphericpressure. This distillation has been found fairly critical to betweenabout 236 and 238 C. When the heads fraction is taken below about 236C., the impurities in the subsequent methylnaphthalenes fraction aresignificantly increased. When the heads fraction is taken above about238 C. the impurities decrease, but the yield of methylnaphthalenesdrops off appreciably and increasingly. A number of compounds withboiling points higher than that of the methylnaphthalenes boil in themethylnaphthalenes range as azeotropes at atmospheric pressure. It hasbeen found unexpectedly that these compounds do not form azeotropes atsubatmospheric pressure and thus a subatmospheric distillation permitsseparation of the methylnaphthalenes therefrom. An atmosphericdistillation alone cannot produce a methylnaphthalenes product from acidwashed coal tar fractions. A subatmospheric distillation alone willproduce a 90% methylnaphthalenes product, but in significantly loweryield.

The vacuum-distilled product comprising a mixture of l-methylnaphthalene(30 to 37%), Z-methylnaphthalene (55 to 65%), and impurities (under10%), flows from the top of column 13 through a pipe 14 to the top ofand downwardly through a chromatographic purification column 15 packedwith silica gel, alumina, activated clay, or any other suitableadsorbing material, preferably silica gel. Such chromatographicpurification is important, since it has been found effective forremoving nitrogenous and other deleterious constituents that interferewith an isomerization, hereinafter to be more fully described.

After about 2.5 parts by weight of the 90% methylnaphthalenes fractionhave passed through the chromatographic column 15 for each part byWeight of silica gel in the column, a desorbent liquid, e.g. methanol,flows from tank 16 through pipe 17 in a total amount of 1.5 parts byweight for each part by weight of adsorbent in column 15. This desorbentliquid, being more polar than the methylnaphthalenes and even more polarthan the impurities which in turn are more polar than themethylnaphthalenes, pushes the methylnaphthalenes and impurities throughthe column ahead of it. Eflluent from the bottom of column 15 first is amethylnaphthalenes fraction, which flows through pipe 18 directly tocontainer 19. By observation or known control means, just prior to flowof impurities, valve 20 is closed and valve 21 is opened to permit flowof impurities and methanol through pipe 22 to still 23, where themethanol and the impurities are separated and flowed respectivelythrough pipes 24 and 25.

After column 15 has drained, it still is saturated with methanol whichrepresents about 25% by weight of the silica gel. The methanol isremoved by heating the column at to C. for about two hours to drive offthe methanol and thus regenerate the column. The column, after cooling,is ready for re-use. Column heating may be effected by coils 26, a steamjacket or other heating means. For continuous operation, additionalchromatographic purification columns, such as columns 15a and 15b, maybe employed in parallel arrangement and with appropriate valves andpiping, as illus trated. In addition to methanol, the desorbant liquidmay be chosen from other highly polar solvents such as ethanol, propanoland isopropanol. Methanol is preferred because of its low boiling point.

As described, the chromatographically purified methylnaphthalenes flowfrom the bottom of column 15 through a pipe 18 as a mixture comprisingabout 95 percent methylnaphthalenes and percent impurities, to acontainer 19 where they may be admixed with an isomerization productflowing thereto through pipe 26, as will be more fully described. Fromcontainer 19, the admixture flows through pipe 27 to a crystallizer 28wherein the methyl naphthalenes are cooled to between about C. and C.,which is below the freezing point of Z-methylnaphthalene (34 C.) butabove the freezing point of l-methylnaphthalene (-3l C.). MostlyZ-methylnaphthalene and some l-methylnaphthalene crystallize out in acold slurry of the crystals in a mother liquor comprising about 47 to 50percent of Z-methylnaphthalene and 39 to 43 percent ofl-methylnaphthalene. The slurry flows from the crystallizer through apipe 29 to a centrifuge 30, to separate crystals and mother liquor forremoval through pipes 31 and 32 respectively. The crystals compriseabout 90 percent Z-methylnaphthalene, about 5 percentl-methylnaphthalene, with the remainder as impurities.

It is preferred to process the mother liquor to obtain additionalZ-methlynaphthalene by flowing the liquor in pipe 32 to a heater 33wherein it is admixed with hydrogen from pipe 34 and vaporized to atemperature between about 400 and 425 C., and then flowing the vaporsthrough pipe 35 to an isomerization chamber 36, which may be a columnpacked with a suitable catalyst, such as 13 percent alumina on silica.

Hydrogen serves to prolong the life of the catalyst. About 3 volumes to10 volumes hydrogen per volume of methylnaphthalenes vapors may be used,a 5:1 ratio being preferred. The isomerization catalyst becomes spentafter about 7 parts by weight of methylnaphthalenes have been passedthrough one part by weight of catalyst. The catalyst may be regeneratedwith oxygen at about 450 to 500 C.

The vapors from column 36 flow through pipe 26 to container 19, theisomerization serving to increase the Z-methylnaphthalene content of themother liquor from about 47-50% to about 56-60%. As hereinabovedescribed, the isomerization increases the quantity of 2-methylnaphthalene available for recovery by admixture with the productfrom pipe 18 flowing to container 19 and thereafter to crystallizer 28and centrifuge 30 for separation of 2-methylnaphthalene and recycling ofmother liquor for isomerization in column 36.

It is evident that the hereinabove described procedures may be varied,for example, to draw off a product of mixed methylnaphthalenes before orafter any of the process steps, the purity thereof increasing as theprocess steps proceed. The ratio of the weight flow rates in pipes 26and 18 may vary between about 0 to 1 and 2.6 to 1 respectively. It isalso evident that, with intermediate product removal in a cyclicoperation, the impurities in the system increase. As a means forcorrecting this situation, when the vapors in pipe 26 have a contentbelow about 55% of 2-methylnaphthalene, they may be drawn off as avarying side stream or completely by appropriately opening valve 37 topipe 10 and distillation column 11.

It will be apparent that distillation columns 11 and 13, columns 15,15a, 15b and 36, container 19, crystallizer 28, centrifuge 30 and heater33 may be of any design known to effect the desired result. Inparticular, the distillation columns may be bubble-tray towerscontaining to 40 trays. The container 19 may be a lower tower or asimple pot in which the vapor from pipe 26 is bubbled through andcondensed in a pool of liquid. The columns 15, 15a, 15b and 36 may bepacked with an appropriate adsorbent and catalyst respectively.

A specific example of the practice of the invention is set forth below.

A coal tar creosote fraction boiling between about 230 and 260 C. whichwas subsequently washed with dilute sulfuric acid, containing 14.5% ofl-methylnaphthalene and 21.0% of Z-methylnaphthalene, was distilled incolumn 11 to remove a fraction boiling under 237 C. as heads and againdistilled in column 13 at a pressure of about 50 mm. mercury absolutepressure, to separate a fraction boiling up to 145 C. at 50 mm. ofmercury and containing methylnaphthalenes as 33% of l-methylnaphthaleneand 57% of 2-methylnaphthalene. The two columns each had 40 theoreticalplates, and were operated at a 5:1 reflux ratio. Chromatographicpurification followed in column 15, packed with silica gel, the liquidsubsequently used as desorbent for impurities being methanol. Theadmixture in container 19 comprised 2.6 parts isomerizedmethylnaphthalenes from pipe 26 to 1 part of chromatographicallypurified methylnaphthalenes from pipe 18, all parts by weight. Theadmixture flowing from container 19 was crystallized at 10 C., theslurry centrifuged, the mother liquor vaporized in admixture with 5volumes heated hydrogen and isomerized at a temperature of about 425 C.at a contact time of 3 to 4 seconds in column 36 packed with a 15%alumina on silica catalyst. The centrifuged product comprisedZ-methylnaphthalene (90%) pure in a 70% yield of methylnaphthalenesoriginally. present and corresponding to a 106% yield of the2-methylnaphthalene originally present.

In the above example, the product from distillation column 13 was amixture of methylnaphthalenes (90% pure) in an 82.5% yield ofmethylnaphthalenes originally present. From chromatographic purificationcolumn 15, a mixture of methylnaphthalenes pure) was obtained in acorresponding 81% yield of the methylnaphthalenes originally present.From the centrifuge 30, when the isomerization step was omitted, therewas obtained Z-methylnaphthalene (90% pure) in a 24% yield and a motherliquor of mixed methylnaphthalenes (95 pure) in a 57% yield ofmethylnaphthalenes originally present.

While the above example illustrates a preferred method of operation,other conditions of operation may be used without departing from thespirit of the invention. The distillation column 11 of 20 to 40theoretical plates may be operated at substantially atmospheric pressureand at reflux ratios from about 1:1 to 5:1. The distillation column 13may be operated at pressures from about 1 to 250 mm. mercury absolutepressure and at reflux ratios as low as 3: 1. The chromatographicadsorbent may be alumina, activated clay or other adsorbent as well assilica gel. The crystallization may be effected at about 5 C. to about25 C., the preferred range being 10 to 15 C. In the isomerization, thetemperature may be from about 375 to 505 C., 400 to 425 C. beingpreferred. The catalyst may be a silica-alumina mixture of about 94 to75% silica and 6 to 25% alumina. The contact time may be 1 to 20seconds, and the hydrogen admixture may be in a ratio between about 3:1and 10:1, 5:1 being preferred.

The invention is characterized by several distinct advantages that makeit possible to recover better yields of high-purity products from coaltar than has been possible heretofore. In the first place, a product ofhigher purity results from the elimination of azeotropes and azeotropeformers in the atmospheric and subatmospheric pressure distillations ofan acid-washed, coal tar fraction boiling from about 230 to 260 C. atsubstantially atmospheric pressure. Secondly, the chromatographicpurification permits separation of nitrogenous and other impurities.More important, this separation is essential, for otherwise theisomerization of l-methylnaphthalene to 2- methylnaphthalene could notbe elfected with any degree of efficiency.

Although we have disclosed herein the preferred practice of ourinvention, we intend to cover as well any change or modification thereinwhich may be made without departing from the spirit and scope of theinvention.

We claim:

1. A method for recovering 2-methylnaphtl1alene from an acid-washed,coal tar fraction boiling between about 230 and 260 C. and containingmethylnaphthalenes in admixture as 1-methylnaphthalene and2-methylnaphthalene, which consists in distilling said coal tar fractionat atmospheric pressure and removing therefrom material boiling belowabout 237 C., further distilling the remaining fraction boiling aboveabout 237 C., between about 1 mm. and 250 mm. mercury absolute pressureand collecting a fraction containing about 90 percentmethylnaphthalenes, passing said collected fraction through achromatographic adsorbent chosen from the group consisting of silicagel, alumina and activated clay, cooling purified methylnaphthaleneseffluent from said chromatographic adsorbent to between about +5 and 25C. and separating crystals of substantially pure 2- methylnaphthalenefrom mother liquor, heating said mother liquor and hydrogen to a vapor,passing said vapor into contact with a silica-alumina catalyst at anisomerization temperature between about 375 and 505 C. and recycling theresulting isomerized product to said cooling step.

'2. A method as defined in claim 1, characterized by following saidchromatographic adsorption step by passing a desorbent fluid more polarthan the constituents in said collected fraction through said adsorbentand recovering further purified methylnaphthalenes efiluent therefrom.

3. A method as defined in claim 2 characterized by using methanol asdesorbent fluid.

4. A method as defined in claim 1, characterized by crystallizing saidpurified methylnaphthalenes efiluent to between about 10 and -15 C.

5. A method as defined in claim 1, characterized by recycling a portionof said isomerized product to said atmospheric pressure distillationstep.

6. A method for recovering Z-methylnaphthalene from an acid-Washed, coaltar fraction boiling between about 230 and 260 C. and containingmethylnaphthalenes in admixture as l-methylnaphthalene and2-methylnaphthalone, which consists in distilling at a reflux ratiobetween about 1 to 1 and 5 to 1 said coal tar fraction at substantiallyatmospheric pressure and removing therefrom material boiling below about237 C., further distilling said fraction at a reflux ratio between about1 to 1 and 5 to 1 and collecting a fraction boiling up to about 145 C.at about 50 mm. mercury absolute pressure, passing said collectedfraction through silica gel, desorbing said silica gel by passingmethanol therethrough and collecting further purified methylnaphthalenesefiluent product therefrom, cooling combined product from the adsorptionand desorption steps to about 15 C. and separating crystals ofsubstantially pure Z-methylnaphthalene from mother liquor, heatingmother liquor and hydrogen in a ratio of about 1 to 5 respectively toabout 425 C., passing said heated admixture over a silica-aluminacatalyst and thereafter recycling the resulting isomerized product tosaid cooling step.

References Cited in the file of this patent UNITED STATES PATENTS2,920,115 Friedman Ian. 5, 1960 OTHER REFERENCES Coulson: Society ofChemical Industry Journal (Trans), May 1941, vol. 60, pages 123-126..

Cullinane et al.: Nature, 1948, vol. 161., page 690.

1. A METHOD FOR RECOVERING 2-METHYLNAPHTHALENE FROM AN ACID-WASHED, COALTAR FRACTION BOILING BETWEEN ABOUT 230* AND 260*C. AND CONTAININGMETHYLNAPHTHALENES IN ADMIXTURE AS 1-METHYLNAPHTHALENE AND2-METHYLNAPHTHALENE, WHICH CONSISTS IN DISTILLING SAID COAL TAR FRACTIONAT ATMOSPHERIC PRESSURE AND REMOVING THEREFROM MATERIAL BOILING BELOWABOUT 237*C., FURTHER DISTILLING THE REMAINING FRACTION BOILING ABOVEABOUT 237*C., BETWEEN ABOUT 1 MM. AND 250 MM. MERCURY ABSOLUTE PRESSUREAND COLLECTING A FRACTION CONTAINING ABOUT 90 PERCENTMETHYLNAPHTHALENES, PASSING SAID COLLECTED FRACTION THROUGH ACHROMATOGRAPHIC ADSORBENT CHOSEN FROM THE GROUP CONSISTING OF SILICAGEL, ALUMINA AND ACTIVATED CLAY, COOLING PURIFIED METHYLNAPHTHALENESEFFLUENT FROM SAID CHROMATOGRAPHIC ADSORBENT TO BETWEEN ABOUT +5* AND-25*C. AND SEPARATING CRYSTALS OF SUBSTANTIALLY PURE 2METHYLNAPHTHALENEFROM MOTHER LIQUOR, HEATING SAID MOTHER LIQUOR AND HYDROGEN TO A VAPOR,PASSING SAID VAPOR INTO CONTACT WITH A SILICA-ALUMINA CATALYST AT ANISOMERIZATION TEMPERATURE BETWEEN ABOUT 375* AND 505* C. AND RECYCLINGTHE RESULTING ISOMERIZED PRODUCT TO SAID COOLING STEP.